In a homogeneous charge compression ignition (‘HCCI’) engine, combustion of a cylinder charge occurs spontaneously throughout the entire combustion chamber volume. The homogeneously mixed cylinder charge is auto-ignited as the cylinder charge is compressed and its temperature increases.
The combustion process in an HCCI engine depends strongly on factors such as cylinder charge composition, temperature, and pressure at the intake valve closing. Hence, the control inputs to the engine, for example, fuel mass and injection timing and intake/exhaust valve profile, must be carefully coordinated to ensure robust auto-ignition combustion. Generally speaking, for best fuel economy, an HCCI engine operates unthrottled and with a lean air-fuel mixture. Further, in an HCCI engine using exhaust recompression valve strategy, the cylinder charge temperature is controlled by trapping different amount of the hot residual gas from the previous cycle by varying the exhaust valve close timing. The opening timing of the intake valve is delayed from normal to a later time preferably symmetrical to the exhaust valve closing timing about top-dead-center (TDC) intake. Both the cylinder charge composition and temperature are strongly affected by the exhaust valve closing timing. In particular, more hot residual gas from a previous cycle can be retained with earlier closing of the exhaust valve which leaves less room for incoming fresh air mass. The net effects are higher cylinder charge temperature and lower cylinder oxygen concentration. In the exhaust recompression strategy, the exhaust valve closing timing (thereby, the intake valve opening timing) is typically quantified by valve overlap which is a negative number, and the Negative Valve Overlap (NVO) is defined as the duration in crank angle between exhaust valve closing and intake valve opening.
In addition to a valve control strategy, there must be a suitable fuel injection strategy for stable combustion. For example, at a low fueling rate (for example, fueling rate<7 mg/cycle at 1000 rpm), the cylinder charge may not be hot enough for a stable auto-ignited combustion in spite of the highest value of NVO allowed, leading to a partial-burn or misfire. One way to increase the charge temperature is to pre-inject a small amount of fuel when a piston approaches intake top-dead-center (TDC) during the recompression. A portion of the pre-injected fuel reforms due to high pressure and temperature during the recompression, and releases heat energy, increasing the cylinder charge temperature enough for successful auto-ignited combustion of the combustion charge resulting from the main fuel injection. The amount of auto-thermal fuel reforming is based upon the pre-injection mass and timing, typically with fuel reforming increasing with earlier pre-injection timing and greater pre-injection fuel mass.
It is important to precisely control the amount of fuel reforming because excessive fuel reforming decreases the overall fuel economy, and lack of fuel reforming may result in combustion instability. With even lower engine load (and therefore lower temperature in the cylinder) reforming of a portion of the fuel during recompression may not be enough to trigger auto-ignition. In this operating range (near idle operation) the main part of the fuel mass is injected late in the main compression rather than during intake. The stratified part of the fuel is ignited by a spark and compresses the remaining fuel-air mixture further to reach auto-ignition. The amount of injected fuel that can be reformed is governed by recompression temperature, pressure, and oxygen availability which depend strongly on engine operation of previous cycle. Thus, better or more robust low load HCCI combustion could be realized if the amount of fuel reforming from cycle to cycle is closely monitored and controlled. Precise fuel reforming control improves combustion performance because excessive reforming decreases fuel economy whereas lack of fuel reforming may result in combustion instability.
Effective control of the reforming process requires accurate estimation of the degree of reforming. A method is known that estimates the amount of fuel reforming using the unique characteristic of UEGO (Universal Exhaust Gas Oxygen) sensor. A control strategy is also known to indirectly control the amount of fuel reforming in an HCCI engine by monitoring engine operating conditions including intake mass air flow and exhaust air/fuel ratio, controlling negative valve overlap to control intake airflow to achieve a desired actual air-fuel ratio for a given fueling rate, and adjusting timing of pre-injection of fuel to control the measured air-fuel ratio to a desired second air/fuel ratio smaller than the desired actual air-fuel ratio. However, conditions within the combustion chamber necessary for proper reforming can vary from combustion cycle to combustion cycle, and known methods to estimate reforming are either overly computer intensive or estimate reforming requirements based on historical data.